1. Technical Field
The present invention relates generally to broadband phase shifting circuits for generating quadrature RF and microwave signals, and more particularly to a stabilized quadrature RC/CR phase shifting network.
2. Background Art
The majority of modern communications systems use complex modulation formats to improve spectral efficiency. Complex modulation entails modulating both of the available degrees of freedom of an electrical signal, namely amplitude and phase, to convey information. Although it is possible to modulate in a polar coordinate system (“polar modulation”) with separate amplitude and phase modulators, the different circuit topologies import different signal dynamics, and modulation accuracy suffers.
It is more common and economical in practice to modulate within a rectangular coordinate system, meaning that the information is cast into real and imaginary parts instead of amplitude and phase. The prevalent lexicon is to replace “real” and “imaginary” with “I” (for “In-phase”) and “Q” (for “Quadrature”), and the corresponding modulators are referred to as IQ modulators. An advantage of an IQ modulator over a polar modulator is that the I and Q information signal paths use identical circuitry, with identical dynamics and thus better modulation accuracy. This is in contrast to the separate AM and PM modulators of a polar modulator, comprising dramatically different types of circuits with different dynamics.
A quadrature phaseshifter operating at the “carrier” or “local oscillator” (“LO”) frequencies is a required part of an IQ modulator, and in many other functional blocks in radios, especially modern radios using complex modulation and/or complex architectures. In these applications, especially ones using complex architectures, phaseshifter accuracy requirements are extreme for both matched amplitudes and exact quadrature-phase, typically amounting to tenth's of a percent and fractional degrees, respectively, and no IC process is capable of that level of performance. However, both errors can be compensated for in a calibration step, and the compensation is actually fairly easy. The hard part is holding the phaseshifter stable enough (both amplitude-difference and quadrature-phase) in between calibrations such that the compensations remain valid. Negative feedback is the answer, as might be expected.
Even so, errors still exist in IQ modulators. Errors may be classified as either linear errors or nonlinear errors. Linear errors are those that can be removed with a single correction value, independent of the information signal. The four linear errors of an IQ modulator are quadrature-phase error, gain ratio error, Ioffset error, and Qoffset error. Nonlinear errors, on the other hand, are errors that are a function of the informational signal itself. As corrections to nonlinear errors are complicated, the nonlinear errors are simply best avoided by using standard design techniques.
FIG. 1 shows a very common prior art narrowband RC/CR phaseshifter circuit. The advantage of such an RC/CR phaseshifter is that outputs are in quadrature at all frequencies. The disadvantage is that amplitudes are equal at only one frequency and diverge elsewhere. It is a narrowband circuit.
Another prior art quadrature RC/CR phaseshift circuit is shown in U.S. Pat. No. 6,310,502, to Klier. The device taught in the '502 patent uses variable resistors and an output quadrature-phase detector feeding an error signal back to the variable resistor in the high pass branch. The low pass branch variable resistor is set to a fixed value per a frequency-to-voltage lookup table and DAC such that its cutoff frequency approximately equals the operating frequency. However, before any difference between cutoff and operating frequencies causes mismatched output amplitudes (as will happen in the circuit shown in Klier, and even worse, since one of the variable resistors is fixed), the quadrature output phase negative feedback will do nothing to aid in stabilizing output amplitude difference via the normal linkage between amplitude and phase in electrical circuits. The present inventor has demonstrated that maintaining equal amplitudes is just as important (if not more so) in maintaining quadrature phase.
Another quadrature phaseshift circuit is the prior art circuit of Magoon et al, shown in U.S. Pat. No. 6,658,066. Magoon et al append extra variable resistance elements at the outputs of RC/CR networks (both single-ended and differential) which are controlled by an output quadrature-phase detector in a negative feedback fashion to establish and maintain accurate quadrature-phase. Magoon et al do not however attend to establishing or maintaining equal output amplitudes. The method taught in Magoon et al is embodied within a VLSI IC that provides the overhead for frequent calibrations that compensate for drift or disturbance inputs.
U.S. Pat. Nos. 5,694,063 and 5,644,260, each to DaSilva (referred to collectively herein as “DaSilva”) show a prior art solution to the divergent output amplitude problem. This solution uses a negative feedback circuit and is illustrated in FIG. 2. In this circuit, if the output amplitudes were to become unequal for any reason (perhaps due to a frequency change, or a “disturbance”), then the detected DC voltage difference is amplified and applied to the variable resistors in a negative feedback fashion to re-equalize the output amplitudes. Automatic amplitude equalization thus occurs over the entire tuning range of the phase shifter. Further, if R equals R and C equals C, output phase will also be in quadrature.
DaSilva includes Voffset to fine-tune exact quadrature-phase to compensate for things that cause phase to deviate from quadrature such as parasitic elements and mismatches between components.
The problem with DaSilva is that in between calibrations quadrature phase is not directly controlled and is susceptible to disturbance inputs and drift.
The foregoing patents reflect the current state of the art of which the present inventor is aware. Reference to, and discussion of, these patents is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.